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The electron configuration of an atom shows how the electrons are arranged in the atom's energy levels. This configuration conveys a lot of important information about an element.

By looking at the electron configuration of selenium, it is possible to determine how many electrons are in each sub-shell. There are five sub-shells, but only four of them are used by naturally occurring elements: s, p, d and f. Each sub-shell accommodates a certain number of electrons.

Sub-shells s, p, d and f hold a maximum of two, six, 10 and 14 electrons, respectively. In selenium, the first energy level has two electrons in sub-shell s. The second energy level holds eight electrons. Two of those electrons are in sub-shell s, while the other six are found in sub-shell p.

The third energy level has a total of 18 electrons. Sub-shell s has two, sub-shell p has six and sub-shell d has 10. Finally, the fourth energy level has a total of six electrons. There are two electrons in sub-shell s and four electrons in sub-shell p. The number of electrons in all of the energy levels adds up to 34.